Observational Constraints on the Formation of the Milky Way's Thick Disk

Young Sun Lee

Michigan State University

The Milky Way's thick disk differs in its structure, kinematics, and
chemical abundances from the thin disk. These discrepancies
imply that the thick disk has experienced different formation and
evolution history from the thin disk. The currently discussed mechanisms
for the thick-disk formation are dynamical heating of pre-exising thin
disk by satellite mergers, accretion of satellite galaxies, chaotic mergers of
gas-rich systems, and radial migration of stars.

In this talk, using [alpha/Fe] ratios obtained for a large
sample of low-resolution (R~2000) spectra of G-type dwarf stars
from the Sloan Extension for Galactic Understanding
and Exploration (SEGUE), I divide the sample into likely thin- and
thick-disk populations, and explore the observed correlations of rotational
velocity with metallicity, Galactocentric distance and
distance from the Galactic plane, as well as the orbital eccentricity
distributions for the individual populations. These correlations
are also compared with the predictions from the formation scenarios mentioned
above in order to provide observational constraints.